BRUSSELS—NATO’s foreign ministers ordered an end to civilian and military co-operation with Russia on Tuesday and told their generals and admirals to figure out ways to better protect alliance members that feel threatened by Vladimir Putin’s Kremlin.

The 28-member alliance, the keystone of U.S. and European security since the end of the Second World War, was reacting to its most serious crisis in years: Russia’s unilateral annexation of Ukraine’s Crimean Peninsula, which the U.S. and its allies have condemned as an illegal land grab.

U.S. Secretary of State John Kerry and the other ministers, including Canadian Foreign Affairs Minister John Baird, met behind closed doors at the North Atlantic Treaty Organization headquarters in Brussels. They unanimously agreed Tuesday on a number of measures. A civilian NATO official who attended the meeting and briefed reporters afterward on condition of anonymity said the steps included:

The suspension of “all practical civilian and military co-operation” between NATO and Russia. NATO officials said ambassadorial-level contacts would remain open to assure a reliable channel of communication.

The possible deployment and reinforcement of military assets in eastern NATO members, such as Poland and the Baltic states, which feel menaced by Moscow’s latest actions.

A possible increase of readiness levels for the NATO rapid response force.

A possible review of NATO’s crisis response plans, as well as its military training and exercise schedules.

NATO Supreme Commander Gen. Phil Breedlove and his subordinates will draw up the proposals within a few weeks and submit them to political leaders for their approval, the NATO official said.

Baird said the NATO decision is just one step in a long journey to end the Ukraine crisis, The Canadian Press reports.

“We’ve got a whole range of options that we’re taking,” Baird said from Brussels. “Obviously, we want to stand in solidarity with our NATO allies, and beef up tactical support, co-operation and efforts.

“There’s no one action which is going to get the change we want. This is just one more step in a long journey to say that it’s unacceptable that in 2014 one man in the Kremlin can try to rewrite the boundaries of Europe.”

An estimated 35,000 to 40,000 Russian troops equipped with tanks, other armoured vehicles and fixed- and rotary-wing aircraft remained positioned near the border with Ukraine, a NATO military official told The Associated Press on Tuesday, speaking on condition of anonymity.

The official described the Russian buildup as “a complete combat force” that was highly threatening to Ukraine.

Meanwhile, the U.S. Congress is sending President Barack Obama a bill to provide $1 billion (U.S.) in loan guarantees to Ukraine and take punitive measures against Russia for its annexation of Crimea.

The bill, passed with bipartisan support, was a way for Congress to denounce Russia’s move and express support for Kyiv.The House voted overwhelmingly to pass it Tuesday. The bill is also aimed at discouraging any further actions that Putin might be contemplating in the region.